1. Field of the Invention
This disclosure is related to the field of the disposition of by-products from wet limestone or lime Flue Gas Desulfurization (FGD), and specifically to the encapsulation of FGD purge water in combination with other coal-fired plant by-products and a fixation agent as part of a low permeability by-product with reduced leachability of pollutants of concern.
2. Description of Related Art
Coal is one of the most common sources of fuel in the electric generation industry and in other industries that rely on a source of heat in order to power the machines of industry. Coal has been a common fuel since the industrial revolution and is not about to be replaced in the near term future. While coal-fired plants are common, there have recently arisen concerns as to the by-products produced from combustion processes generally, and specifically burning coal.
One major issue with the by-products of coal combustion is inhibiting the release of sulfur dioxide (SO2). Emission standards from a wide variety of United States Environmental Protection Agency (US EPA) driven regulatory requirements under the Clean Air Act have required a large number of coal-fired plants to retrofit and install “scrubbers” to capture sulfur dioxide from their flue gas. This process is called Flue Gas Desulfurization (FGD). One common technology for FGD is wet Lime or Limestone scrubbers where the calcium source (lime or limestone) is mixed in a slurry and introduced to the gas stream in a large reactor to “scrub” the SO2 from the gas stream. Most coals burned in these plants contain small concentrations of chlorine. The chlorine is readily scrubbed from the gas within the FGD system. As the calcium slurry is recirculated in the scrubber, these chlorine compounds that have been removed (“chlorides”) accumulate and, if not controlled, will concentrate in the slurry. As chloride concentrations rise, they begin to cause operational issues with the FGD system due to their corrosive nature which increases with concentration. If left uncontrolled, these chlorides can attack even the most exotic and expensive metal alloys causing major damage to an FGD system. In order to control this chloride corrosion phenomenon, it is common practice to control the level of chlorides present in the scrubber slurry by removing or “purging” a portion of the scrubber slurry. This process is often referred to as a “chloride purge”.
The FGD purge stream is an aqueous solution and generally contains a wide variety of pollutants. It, includes gypsum, along with heavy metals, chlorides, magnesium and dissolved organics. In many applications, FGD purge water is first treated by dewatering to separate synthetic gypsum cake which can be a valuable secondary product. The remaining FGD purge water is then recycled back to the scrubber. A portion of this water (still containing dissolved chlorides) is removed from the recycle stream (the “chloride purge”), and is subjected to various forms of water treatment (as required) to reduce or eliminate dissolved metals and other contaminants of concern prior to discharge back into the environment in accordance with the applicable permits and laws.
Fly ash is also a common by-product of coal-fired plants, although it is produced in most other forms of combustion as well. Fly ash is entrained as a component of flue gas and is generally captured by electrostatic precipitators, baghouses, or other particle filtration equipment prior to flue gas exiting the stack. Fly ash also commonly includes elevated concentrations of heavy metals as well as silicas and alumina compounds. Once captured, fly ash is also generally ponded or landfilled to allow settling and containment of the fly ash.
Using current wastewater treatment technologies does not remove all the contaminants of potential concern. Some contaminants, such as selenium, are very difficult to remove. US EPA has indicated an intent to further regulate this discharge in order to address some of these concerns. Treatment systems to deal with these new concerns will be very expensive both in their capital cost and often in their operating costs.